Okay, I just submitted it as a false positive. I'd never had one before.
I don't know how to find the different exclusion lists. I'm given an opportunity to add it from the chest, as well as from the dialog box that pops up to tell me that a suspicious item has been detected. While I assume that it's the file shield that's detecting it, it's not obvious to me.
Part of my problem is that I've used Avast since switching from Avira over a year ago, and it Just Works. I've not had to fiddle with it, so haven't learned much about its details.
Guessing from what you told me, I just now tried using the exclusion option from the dialog box, as opposed to the chest, where I was doing it previously. Amazingly (to me), that worked! I have ACDSee back!
I still don't understand why it works from there, and only appears to work from the virus chest, but I'll take the victory and run. Thank you for your helpful hints!